Many of the new, alternative refrigerants are ‘blends’, which have two or three components, developed for existing and new plants as comparable alternatives to the refrigerants being replaced. They are ‘zeotropes’ with varying evaporating or condensing temperatures in the latent heat of vaporization phase, referred to as the ‘temperature glide’. Figure 3.3 shows the variation in evaporating and condensing temperatures.

To compare the performance between single component refrigerants and blends it will be necessary to specify the evaporating temperature of the blend to point A on the diagram and the condensing temperature to point B.

The temperature glide can be used to advantage in improving plant performance, by correct design of the heat exchangers. A problem associated with blends is that refrigerant leakage results in a change in the component concentration of the refrigerant. However, tests indicate that small changes in concentration (say less than 10%) have a negligible effect on plant performance.

The following recommendations apply to the use of blends:

• The plant must always be charged with liquid refrigerant, or the component concentrations will shift.
• Since most blends contain at least one flammable component, the entry of air into the system must be avoided.
• Blends which have a large temperature glide, greater than 5K, should not be used for flooded-type evaporators.